Recovering hydrocarbons from subterranean zones typically involves drilling wellbores.
Wellbores are made using surface-located drilling equipment which drives a drill string that eventually extends from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. Drilling fluid usually in the form of a drilling “mud” is typically pumped through the drill string. The drilling fluid cools and lubricates the drill bit and also carries cuttings back to the surface. Drilling fluid may also be used to help control bottom hole pressure to inhibit hydrocarbon influx from the formation into the wellbore and potential blow out at the surface.
Bottom hole assembly (BHA) is the name given to the equipment at the terminal end of a drill string. In addition to a drill bit a BHA may comprise elements such as: apparatus for steering the direction of the drilling (e.g. a steerable downhole mud motor or rotary steerable system); sensors for measuring properties of the surrounding geological formations (e.g. sensors for use in well logging); sensors for measuring downhole conditions as drilling progresses; one or more systems for telemetry of data to the surface; stabilizers; heavy weight drill collars, pulsers and the like. The BHA is typically advanced into the wellbore by a string of metallic tubulars (drill pipe).
Modern drilling systems may include any of a wide range of electronics systems in the BHA or at other downhole locations. Such electronics systems may be packaged as part of a downhole probe. A downhole probe may comprise any active mechanical, electronic, and/or electromechanical system that operates downhole. A probe may provide any of a wide range of functions including, without limitation, data acquisition, measuring properties of the surrounding geological formations (e.g. well logging), measuring downhole conditions as drilling progresses, controlling downhole equipment, monitoring status of downhole equipment, measuring properties of downhole fluids and the like. A probe may comprise one or more systems for: telemetry of data to the surface; collecting data by way of sensors (e.g. sensors for use in well logging) that may include one or more of vibration sensors, magnetometers, inclinometers, accelerometers, nuclear particle detectors, electromagnetic detectors, acoustic detectors, and others; acquiring images; measuring fluid flow; determining directions; emitting signals, particles or fields for detection by other devices; interfacing to other downhole equipment; sampling downhole fluids, etc. Some downhole probes are highly specialized and expensive.
Various radioactive elements occur naturally in the earth. Different types of geological formations typically contain different amounts of such radioactive elements and therefore emit different amounts and different spectra of natural gamma radiation. Measuring gamma-radiation with a detector located inside a downhole probe within a borehole is a common operation in well logging. Natural gamma-rays are emitted when materials such as thorium, uranium and potassium (Th, U, K) undergo radioactive decay. Each element emits gamma-radiation at characteristic energies resulting in a characteristic gamma radiation spectrum . Measuring natural gamma-radiation is particularly useful in exploiting oil and gas resources because it is believed that the concentrations of Th, U, K taken individually or in combination are a good indication as to the characteristics of formations surrounding the borehole which may affect the availability for extraction of hydrocarbons. Such characteristics may include, for example, the presence, type, and volume of shale or clay.
Gamma-radiation is attenuated in passing through the walls of a drill collar. Therefore, the sensitivity of a gamma-radiation detector located inside a downhole probe within a drill collar is reduced. Another source of attenuation for gamma-radiation measurements is drilling fluid surrounding the downhole probe.
Downhole conditions can be harsh. Exposure to these harsh conditions, which can include vibrations, turbulence and pulsations in the flow of drilling fluid, shocks, and immersion in various drilling fluids at high pressures can shorten the lifespan of downhole probes and can cause failure of the electronics and electromechanical systems housed within downhole probes.
The following references describe technology that may be of interest to those reading this disclosure:                U.S. Pat. No. 6,300,624;        U.S. Pat. No. 6,666,285;        U.S. Pat. No. 6,944,548;        U.S. Pat. No. 6,975,243;        U.S. Pat. No. 7,566,235;        U.S. Pat. No. 7,685,732;        U.S. Pat. No. 7,897,915;        US 2013/0105678;        CA 2549588;        CA 2565898;        CA 2706861;        WO 2008/112331; and,        WO 2008/116077.        
There remains a need for cost-effective and easily serviceable ways to house electronics and electromechanical systems in downhole drilling operations, which may include gamma-radiation detectors and other electronics systems of a wide range of types. There is also a continual need to provide alternative systems for downhole gamma-radiation measurement.